WO08135789 and EP2112375 from the same patent family disclose a method of detecting formation of ice on the blades of a wind turbine, wherein the blades are provided with fibre strain sensors for measuring mechanical strain of the wind turbine blades. The documents describe that the fibre strain sensor may be calibrated by use of a separate calibration procedure, wherein the pitch angle are positioned so that maximum component of the blade weight acts on flapwise and edgewise sensors, respectively. The rotor of the wind turbine is then rotated at a constant speed in order to minimise aerodynamic and centrifugal forces and with no electrical load on the generator of the wind turbine. The calibration procedure is carried out with the blades in a fixed pitch position for several revolutions of the wind turbine rotor. A sinusoidal signal can be extracted from the data using a bandpass filter locked to the rotational frequency of the wind turbine.
WO08020239 and WO08020240 describe a method of manufacturing a wind turbine with a number of wind turbine blades provided with optical fibre Bragg grating sensors.
WO09056869 describes a method of monitoring strain on a wind turbine blade, wherein at least three strain sensors are located on one of the structural components of the wind turbine blade in a transverse plane thereof, wherein strain signals from the strain sensors are resolved into strain measurements in two orthogonal directions.
DE102006036157 discloses a method of calibrating a sensor of a wind turbine. The calibration is carried out in a separate process once an initiation signal has been sent. The calibration is carried out by positioning the rotor in a brake position with a blade in a horizontal position and letting the blade run through a pitch cycle, after which the data signal from the sensor is analysed. Alternatively, the blade is maintained at a fixed pitch angle while data is collected for several rotations of the rotor.
In order to obtain reliable measurements, the load sensors must be calibrated. This can be done in the factory, but is relatively costly. If for some reason the load sensors need to be re-calibrated after some operation on the turbine, it would be very expensive to dismount the blades and carry them to the factory for calibration, which is especially apparent for off-shore wind turbines. Therefore, there is a demand for a possibility of calibrating the sensors on-site and preferably while the blade is mounted on the wind turbine and more preferably during normal operation of the wind turbine.